1. Field of the Invention
This invention is in the field of photocathodes and more specifically to glass sealed transmission-made negative electron affinity (NEA) GaAs photocathodes.
2. Description of the Prior Art
Various types of photocathodes are known in the art, and usual for these types are multialkali types. However, NEA photocathodes are capable of luminous sensitivities far in excess of the valves exhibited by multialkali photocathodes. In order to obtain a high-performance NEA GaAs transmission photocathode it is necessary to have a uniformly thin and blemish-free p+ GaAs layer with a long minority carrier diffusion length (high crystal quality) on a transparent substrate with a low recombination velocity (passivated) at the photon input interface. Additionally, the photocathode surface must be readily heat cleaned in vacuum at about 600.degree. C. so that it can be activated with cesuim (Cs) and oxygen (O.sub.2) to a state of NEA. A number of approaches have been tried in attempts to realize these requirements, including (a) GaAs/Al.sub.2 O.sub.3, (b) GaAs/GaP, (c) GaAs/GaAsP/GaP, (d) GaAs/GaAlAs/GaP, (e) GaAs/GaAlAs/GaAs (a rim-supported structure, (f) GaAs/GaAlAs/GaP (hybrid structure), and (g) GaAs/GaAlAs/glass (glass-sealed structure). All these structures are prepared using any one of the following growth techniques: vapor phase epitaxy (using halogen transport); liquid phase epitaxy; or a combination of both vapor and liquid phase epitaxy (hybrid epitaxy). With the exception of (a) all the other methods use either GaAs or GaP as the seed crystal for epitaxial growth. Sapphire (Al.sub.2 O.sub.3) is not suitable as a seed crystal due to the inferior photocathode crystal quality resulting from poor lattice match. Unfortunately, GaAs and GaP seed crystals are expensive and of limited diameter. In addition, both GaAs and GaP are currently manufactured with inferior crystal quality as compared with the Ge crystals used with the instant invention.